The human spine is a flexible structure comprised of thirty-three vertebrae separated by intervertebral discs. The intervertebral discs act as shock absorbers cushioning adjacent vertebrae and permitting bending between the vertebrae. Generally, an intervertebral disc is comprised of a nucleus pulposus, an annulus fibrosis, and end plates. The nucleus pulposus comprises an inner gel-like core which occupies 25-40% of the disc's total cross-sectional area. The annulus fibrosis is a collagen fiber composite structure that surrounds the nucleus pulposus and resists hoop, torsional and bending stresses applied to the disc. The cartilaginous end plates separate the disc from the vertebrae on either side of the disc.
Currently, back pain remains a major public health problem, especially among aged people. Persistent and severe back pain may debilitate and disable the sufferer. A common cause of such pain is frequently the result of intervertebral disc abnormalities. For example, damage to one or more of the vertebrae and/or one or more discs may result from trauma, exertion, injury, illness, or abuse. More specifically, disorders of the vertebrae and discs may include, but are not limited, to: (1) disruption of the disc annulus such as annular fissures; (2) chronic inflammation of the disc; (3) localized disc herniations with contained or escaped extrusions; and (4) instability of the vertebrae surrounding the disc.
Various approaches have been developed to treat back pain. For example, minor back pain may be treated with medication and other non-invasive therapies. However, severe back pain often necessitates the removal of at least a portion of the damaged and/or malfunctioning back component. Should the disc become ruptured, the ruptured disc may be surgically removed and the two adjacent vertebrae proximate to the removed disc may be permitted to fuse together. Alternatively, the end plates of two adjacent vertebrae may be fused posterior-laterally by screws or other fusing devices. While these fusion procedures have proven successful in treating some intervertebral dysfunctions, several shortcomings have been discovered. For example, pseudoarthrosis may result from such posterior fusion procedures.
In light of the foregoing, there is an ongoing need for an implantable intervertebral device capable of simulating the natural movement of the vertebrae.